The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventor(s), to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Direct current (DC) motors, including brushless DC motors (BLDC), commonly require electronic circuitry to control commutation for driving the motor. There are multiple ways to drive a DC motor. One method for driving a DC motor is to use one or more Hall sensors to detect a motor pole position to determine how and when to drive appropriate phases (i.e., terminals) of the motor to keep the motor spinning.
FIG. 1 illustrates an example environment 100 comprising a simplified model of a 3 phase motor 102. Motor 102 is illustrated as having three coils 104, 106 and 108 arranged in three directions corresponding to terminals for phases A 110, B 112 and C 114, respectively. In an ideal 3 phase motor, terminals for phases A, B and C are positioned 120° apart. Rotor 118 is represented as a bar magnet with its rotary axis at the intersection of phases A 110, B 112 and C 114, and perpendicular to the plane of the axes associated with the phases. The rotational position of rotor 118 can be controlled by driving a configuration of currents through coils 104, 106 and 108 at the terminals of phases 110, 112 and 114, respectively. FIG. 1 illustrates motor 102 equipped with Hall sensors 120, 122 and 124 used to detect actual positions (i.e., pole positions) of rotor 118. The actual pole positions are commonly used by control circuitry to determine how to drive phases A, B and C. In an ideal motor, Hall sensor 120 would be located rotationally half-way between the terminals of phases A 110 and B 112; Hall sensor 122 would be located rotationally half-way between the terminals of phases B 112 and C 114; and Hall sensor 124 would be located rotationally half-way between the terminals of phases C 114 and 110.
The motion of rotor 118 induces alternating voltages called Back Electro-Motive Force (BEMF) within coils 104, 106 and 108. The amplitude of the BEMF voltage is generally proportional to the angular velocity of rotor 118. Hall sensors are precisely mounted in such a way that a zero crossing of a BEMF voltage waveform occurs as close as possible to a zero crossing of the Hall sensor signal associated with a corresponding coil.
Another method for controlling a DC motor is a “sensor-less method” that does not rely on the use of precisely positioned Hall sensors. In the “sensor-less method”, the drive of motor 102 on one or more terminals A 110, B 112 and C 114 is stopped for a short period, commonly referred to as a “window”, in order to monitor the BEMF voltage of the motor. The zero crossing of the BEMF voltage on one or more phases of the motor will provide information on the pole position, which is commonly used to determine how to drive the appropriate terminals of the motor to keep the motor running.
FIG. 2 illustrates a “sensor-less method” of determining a zero crossing of the BEMF voltage on one or more phases of motor 102. In an ideal 3 phase motor, the BEMF voltages of phases A 110, B 112 and C 114 are sinusoidal and out of phase by 120° relative to each other. To drive motor 102 in synchronization with the BEMF voltage on each phase of motor 102, to maximize efficiency of motor 102, a reference for BEMF signals on each phase may be used. If the BEMF signals on each of phases A 110, B 112 and C 114 are exactly 120° apart relative to each other, and they each have a sinusoidal waveform with a same amplitude, then detection of the BEMF signal on only one phase (e.g., phase A 110) may be used, as BEMF waveforms on other phases may be extrapolated from the zero crossing detected on the one phase.
As shown in FIG. 2, phase A 110 is driven in a windowed fashion, while phases B 112 and C 114 are driven in a continuous, non-interrupted windowless fashion. At points near a zero crossing of the phase A BEMF voltage waveform, as illustrated in FIG. 2, a driving of phase A is stopped to open a window, so that the phase A BEMF voltage may be observed to detect a precise time of its zero crossing. This precise time is used to extrapolate appropriately synchronized continuous driving of phases B 112 and C 114, as well as synchronized windowed driving of phase A 110.